A FEW YEARS AGO, I hosted a meeting for Treasury Secretary Tim Geithner at Facebook. We invitedfifteen executives from across Silicon Valley for breakfast and a discussion about the economy.

Secretary Geithner arrived with four members of his staff, two senior and two more junior, and we allgathered in our one nice conference room. After the usual milling around, I encouraged the attendeesto help themselves to the buffet and take a seat. Our invited guests, mostly men, grabbed plates andfood and sat down at the large conference table. Secretary Geithner’s team, all women, took their foodlast and sat in chairs off to the side of the room. I motioned for the women to come sit at the table,waving them over so they would feel welcomed. They demurred and remained in their seats.

The four women had every right to be at this meeting, but because of their seating choice, theyseemed like spectators rather than participants. I knew I had to say something. So after the meeting, Ipulled them aside to talk. I pointed out that they should have sat at the table even without aninvitation, but when publicly welcomed, they most certainly should have joined. At first, they seemedsurprised, then they agreed.

It was a watershed moment for me. A moment when I witnessed how an internal barrier can alterwomen’s behavior. A moment when I realized that in addition to facing institutional obstacles, womenface a battle from within.

When I gave a TEDTalk on how women can succeed in the workforce, I told this story to illustratehow women hold themselves back, literally choosing to watch from the sidelines. And yet asdisappointed as I was that these women made that choice, I also deeply understood the insecuritiesthat drew them to the side of the room and kept them glued to those chairs.

My senior year of college, I was inducted into the Phi Beta Kappa honor society. At that time,Harvard and Radcliffe had separate chapters, so my ceremony was for women only. The keynotespeaker, Dr. Peggy McIntosh from the Wellesley Centers for Women, gave a talk called “Feeling Likea Fraud.”

She explained that many people, but especially women, feel fraudulent when they arepraised for their accomplishments. Instead of feeling worthy of recognition, they feel undeserving andguilty, as if a mistake has been made. Despite being high achievers, even experts in their fields,women can’t seem to shake the sense that it is only a matter of time until they are found out for whothey really are—impostors with limited skills or abilities.

I thought it was the best speech I had ever heard. I was leaning forward in my chair, noddingvigorously. Carrie Weber, my brilliant and totally-not-a-fraud roommate, was doing the same. At last,someone was articulating exactly how I felt. Every time I was called on in class, I was sure that I wasabout to embarrass myself. Every time I took a test, I was sure that it had gone badly. And every time Ididn’t embarrass myself—or even excelled—I believed that I had fooled everyone yet again. One daysoon, the jig would be up.

At the joint reception that followed the ceremony—an after-party for nerds, so I fit right in—I toldone of my male classmates about Dr. McIntosh’s fantastic speech explaining how we all feel likefrauds. He looked at me, confused, and asked, “Why would that be interesting?” Carrie and I laterjoked that the speech to the men was probably something like “How to Cope in a World Where NotEveryone Is as Smart as You.”

This phenomenon of capable people being plagued by self-doubt has a name—the impostorsyndrome. Both men and women are susceptible to the impostor syndrome, but women tend toexperience it more intensely and be more limited by it.

Even the wildly successful writer and actressTina Fey has admitted to these feelings. She once explained to a British newspaper, “The beauty of theimpostor syndrome is you vacillate between extreme egomania, and a complete feeling of: ‘I’m afraud! Oh god, they’re on to me! I’m a fraud!’ So you just try to ride the egomania when it comes andenjoy it, and then slide through the idea of fraud. Seriously, I’ve just realized that almost everyone is afraud, so I try not to feel too bad about it.”

For women, feeling like a fraud is a symptom of a greater problem. We consistently underestimateourselves. Multiple studies in multiple industries show that women often judge their own performanceas worse than it actually is, while men judge their own performance as better than it actually is.

Assessments of students in a surgery rotation found that when asked to evaluate themselves, thefemale students gave themselves lower scores than the male students despite faculty evaluations thatshowed the women outperformed the men.

A survey of several thousand potential political candidatesrevealed that despite having comparable credentials, the men were about 60 percent more likely tothink that they were “very qualified” to run for political office.

A study of close to one thousandHarvard law students found that in almost every category of skills relevant to practicing law, womengave themselves lower scores than men.

Even worse, when women evaluate themselves in front ofother people or in stereotypically male domains, their underestimations can become even morepronounced.

Ask a man to explain his success and he will typically credit his own innate qualities and skills. Aska woman the same question and she will attribute her success to external factors, insisting she did wellbecause she “worked really hard,” or “got lucky,” or “had help from others.” Men and women alsodiffer when it comes to explaining failure. When a man fails, he points to factors like “didn’t studyenough” or “not interested in the subject matter.” When a woman fails, she is more likely to believe itis due to an inherent lack of ability.

And in situations where a man and a woman each receivenegative feedback, the woman’s self-confidence and self-esteem drop to a much greater degree.

Theinternalization of failure and the insecurity it breeds hurt future performance, so this pattern hasserious long-term consequences.

And it’s not just women who are tough on themselves. Colleagues and the media are also quick tocredit external factors for a woman’s achievements. When Facebook filed to go public, The New YorkTimes ran an article that kindly reminded me—and everyone else—that I had “been lucky” and “hadpowerful mentors along the way.”

Journalists and bloggers rose up to highlight the double standard,pointing out that The New York Times rarely ascribed men’s success to having been lucky. But theTimes didn’t say anything that I had not already told myself a thousand times. At every stage of mycareer, I have attributed my success to luck, hard work, and help from others.

My insecurity began, as most insecurities do, in high school. I attended a big public school inMiami—think Fast Times at Ridgemont High—that was far more concerned with preventing fights inthe halls and keeping drugs out of the bathrooms than with academics. When I was accepted intoHarvard, many of my high school classmates asked me why I would want to go to a school filled withgeeks. Then they would stop short, remember who they were talking to, and sheepishly walk awaywithout waiting for an answer, realizing they already had it.

Freshman year of college was a huge shock for me. First semester, I took a course called TheConcept of the Hero in Hellenic Civilization, which was nicknamed Heroes for Zeroes. I didn’t have aburning desire to study Greek mythology, but it was the easiest way to fulfill the literaturerequirement. The professor began the first lecture by asking which students had read these booksbefore. I whispered to my friend next to me, “What books?” “The Iliad and The Odyssey, of course,”

she replied. Almost every single hand went up. Not mine. The professor then asked, “And who hasread these books in the original?” “What original?” I asked my friend. “Homeric Greek,” she replied.

A good third of the class kept their hands up. It seemed pretty clear that I was one of the zeroes.

A few weeks later, my professor of political philosophy assigned a five-page paper. I was panicked.

Five whole pages! I had only written one paper of that length in high school, and it was a year-longproject. How could anyone write five pages in just one week? I stayed in every night, plugging away,and based on the time I put in, I should have gotten an A for effort. I got a C. It is virtually impossibleto get a C at Harvard if the assignment is turned in. I am not exaggerating—this was the equivalent ofa failing grade. I went to see my dorm proctor, who worked at the admissions office. She told me thatI had been admitted to Harvard for my personality, not my academic potential. Very comforting.

I buckled down, worked harder, and by the end of the semester, I learned how to write five-pagepapers. But no matter how well I did academically, I always felt like I was about to get caught for notreally knowing anything. It wasn’t until I heard the Phi Beta Kappa speech about self-doubt that itstruck me: the real issue was not that I felt like a fraud, but that I could feel something deeply andprofoundly and be completely wrong.

I should have understood that this kind of self-doubt was more common for females from growingup with my brother. David is two years younger than I am and one of the people in the world whom Irespect and love the most. At home, he splits child care duties with his wife fifty-fifty; at work, he’s apediatric neurosurgeon whose days are filled with heart-wrenching life-and-death decisions. Althoughwe had the same upbringing, David has always been more confident. Once, back in high school, weboth had Saturday night dates who canceled on us in the late afternoon. I spent the rest of the weekendmoping around the house, wondering what was wrong with me. David laughed off the rejection,announcing, “That girl missed out on a great thing,” and went off to play basketball with his friends.

Luckily, I had my younger sister, wise and empathetic way beyond her years, to console me.

A few years later, David joined me at college. When I was a senior and he was a sophomore, wetook a class in European intellectual history together. My roommate, Carrie, also took the class, whichwas a huge help since she was a comparative literature major. Carrie went to all of the lectures andread all ten of the assigned books—in the original languages (and by then, I knew what those were). Iwent to almost all of the lectures and read all of the books—in English. David went to two lectures,read one book, and then marched himself up to our room to get tutored for the final exam. We all sattogether for the test, scribbling furiously for three hours in our little blue books. When we walked out,we asked one another how it went. I was upset. I had forgotten to connect the Freudian id toSchopenhauer’s conception of the will. Carrie, too, was concerned and confessed that she hadn’tadequately explained Kant’s distinction between the sublime and the beautiful. We turned to mybrother. How did he feel about the test? “I got the flat one,” he announced. “The flat one?” we asked.

“Yeah,” he said, “the flat A.”

He was right. He did get the flat one. Actually, we all got flat A’s on the exam. My brother was notoverconfident. Carrie and I were overly insecure.

These experiences taught me that I needed to make both an intellectual and an emotionaladjustment. I learned over time that while it was hard to shake feelings of self-doubt, I couldunderstand that there was a distortion. I would never possess my brother’s effortless confidence, but Icould challenge the notion that I was constantly headed for failure. When I felt like I was not capableof doing something, I’d remind myself that I did not fail all of my exams in college. Or even one. Ilearned to undistort the distortion.

We all know supremely confident people who have no right to feel that way. We also all knowpeople who could do so much more if only they believed in themselves. Like so many things, a lack ofconfidence can become a self-fulfilling prophecy. I don’t know how to convince anyone to believedeep down that she is the best person for the job, not even myself. To this day, I joke that I wish Icould spend a few hours feeling as self-confident as my brother. It must feel so, so good—likereceiving a cosmic flat one every day.

When I don’t feel confident, one tactic I’ve learned is that it sometimes helps to fake it. I discoveredthis when I was an aerobics instructor in the 1980s (which meant a silver leotard, leg warmers, and ashiny headband, all of which went perfectly with my big hair). Influenced by the gospel of JaneFonda, aerobics also meant smiling solidly for a full hour. Some days, the smile came naturally. Otherdays, I was in a lousy mood and had to fake it. Yet after an hour of forced smiling, I often feltcheerful.

Many of us have experienced being angry with someone and then having to pretend everything’sgreat in public. My husband, Dave, and I have our moments, and just when we are getting into it, itwill be time to go to a friend’s house for dinner. We put on our “everything’s great” smiles, andamazingly, after a few hours, it often is.

Research backs up this “fake it till you feel it” strategy. One study found that when people assumeda high-power pose (for example, taking up space by spreading their limbs) for just two minutes, theirdominance hormone levels (testosterone) went up and their stress hormone levels (cortisol) wentdown. As a result, they felt more powerful and in charge and showed a greater tolerance for risk. Asimple change in posture led to a significant change in attitude.

I would not suggest that anyone move beyond feeling confident into arrogance or boastfulness. Noone likes that in men or women. But feeling confident—or pretending that you feel confident—isnecessary to reach for opportunities. It’s a cliché, but opportunities are rarely offered; they’re seized.

During the six and a half years I worked at Google, I hired a team of four thousand employees. I didnot know all of them personally, but I knew the top hundred or so. What I noticed over the years wasthat for the most part, the men reached for opportunities much more quickly than the women. Whenwe announced the opening of a new office or the launch of a new project, the men were banging downmy door to explain why they should lead the charge. Men were also more likely to chase a growthopportunity even before a new opening was announced. They were impatient about their owndevelopment and believed that they were capable of doing more. And they were often right—just likemy brother. The women, however, were more cautious about changing roles and seeking out newchallenges. I often found myself trying to persuade them to work in new areas. I have had countlessconversations where women responded to this encouragement by saying, “I’m just not sure I’d begood at that.” Or “That sounds exciting, but I’ve never done anything like it before.” Or “I still have alot to learn in my current role.” I rarely, if ever, heard these kinds of comments from men.

Given how fast the world moves today, grabbing opportunities is more important than ever. Fewmanagers have the time to carefully consider all the applicants for a job, much less convince morereticent people to apply. And increasingly, opportunities are not well defined but, instead, come fromsomeone jumping in to do something. That something then becomes his job.

When I first joined Facebook, I was working with a team to answer the critical question of how bestto grow our business. The conversations were getting heated, with many people arguing their ownpositions strongly. We ended the week without consensus. Dan Rose, leader of our deal team, spentthe weekend gathering market data that allowed us to reframe the conversation in analytics. His effortbroke the logjam. I then expanded Dan’s responsibilities to include product marketing. Takinginitiative pays off. It is hard to visualize someone as a leader if she is always waiting to be told what todo.

Padmasree Warrior, Cisco’s chief technology officer, was asked by The Huffington Post, “What’sthe most important lesson you’ve learned from a mistake you’ve made in the past?” She responded, “Isaid no to a lot of opportunities when I was just starting out because I thought, ‘That’s not what mydegree is in’ or ‘I don’t know about that domain.’ In retrospect, at a certain point it’s your ability tolearn quickly and contribute quickly that matters. One of the things I tell people these days is thatthere is no perfect fit when you’re looking for the next big thing to do. You have to take opportunitiesand make an opportunity fit for you, rather than the other way around. The ability to learn is the mostimportant quality a leader can have.”

Virginia Rometty, IBM’s first female CEO, told the audience at the 2011 Fortune Most PowerfulWomen Summit that early in her career, she was offered a “big job.” She worried that she lacked theproper experience and told the recruiter that she needed to think about it. That night, she discussed theoffer with her husband, who pointed out, “Do you think a man would have ever answered thatquestion that way?”

“What it taught me was you have to be very confident,” Ginni said. “Even though you’re so self-critical inside about what it is you may or may not know. And that, to me, leads to taking risks.”

I continue to be alarmed not just at how we as women fail to put ourselves forward, but also at howwe fail to notice and correct for this gap. And that “we” includes me. A few years ago, I gave a talk ongender issues to a few hundred employees at Facebook. After my speech, I took questions for as longas time permitted. Later that afternoon, I came back to my desk, where a young woman was waiting totalk to me. “I learned something today,” she said. “What?” I asked, feeling good, as I figured she wasabout to tell me how my words had touched her. Instead, she said, “I learned to keep my hand up.”

She explained that toward the end of my talk, I had said that I would take only two more questions. Idid so, and then she put her hand down, along with all of the other women. But several men kept theirhands up. And since hands were still waving in the air, I took more questions—only from the men.

Instead of my words touching her, her words hit me like a ton of bricks. Even though I was giving aspeech on gender issues, I had been blind to one myself.

If we want a world with greater equality, we need to acknowledge that women are less likely tokeep their hands up. We need institutions and individuals to notice and correct for this behavior byencouraging, promoting, and championing more women. And women have to learn to keep theirhands up, because when they lower them, even managers with the best intentions might not notice.

When I first started working for Larry Summers, then chief economist at the World Bank, he wasmarried to a tax attorney, Vicki. He was very supportive of Vicki’s career and used to urge her to “billlike a boy.” His view was that the men considered any time they spent thinking about an issue—eventime in the shower—as billable hours. His wife and her female colleagues, however, would decide thatthey were not at their best on a given day and discount hours they spent at their desks to be fair to theclient. Which lawyers were more valuable to that firm? To make his point, Larry told them the story ofa renowned Harvard Law School professor who was asked by a judge to itemize a bill. The professorresponded that he could not because he was so often thinking about two things at once.

Even now, I’m a long way from mastering the art of feeling confident. In August 2011, Forbes putout its annual World’s 100 Most Powerful Women list.

I’m savvy enough to know that the list wasn’tbased on a scientific formula and that magazines love these features because they generate lots of pageviews as readers click through each name. Still, I was shocked—no, horrified—to learn that Forbesranked me as the fifth most powerful woman in the world, right after German chancellor AngelaMerkel, Secretary of State Hillary Clinton, Brazilian president Dilma Rousseff, and the CEO ofPepsiCo, Indra Nooyi. This put me ahead of First Lady Michelle Obama and Indian politician SoniaGandhi. Absurd. My own mother called to say, “Well, dear, I do think you are very powerful, but I amnot sure you are more powerful than Michelle Obama.” You think?

Far from feeling powerful, I felt embarrassed and exposed. When colleagues at Facebook stoppedme in the halls to say congratulations, I pronounced the list “ridiculous.” When friends posted the linkon Facebook, I asked them to take it down. After a few days, my longtime executive assistant, CamilleHart, summoned me into a conference room and closed the door. This was serious. She told me that Iwas handling the Forbes thing poorly and that I needed to stop subjecting anyone who brought up thelist to a diatribe on its absurdity. I was showing too many people how uncomfortable I felt andrevealing my insecurity. Instead, I needed to simply say, “Thank you.”

We all need colleagues like Camille, who was honest enough to point out my less-than-graciousresponse. She was right. Whether the list was ridiculous or not, I didn’t write it and I didn’t have toreact negatively to it. I doubt a man would have felt so overwhelmed by others’ perception of hispower.

I know that my success comes from hard work, help from others, and being at the right place at theright time. I feel a deep and enduring sense of gratitude to those who have given me opportunities andsupport. I recognize the sheer luck of being born into my family in the United States rather than one ofthe many places in the world where women are denied basic rights. I believe that all of us—men andwomen alike—should acknowledge good fortune and thank the people who have helped us. No oneaccomplishes anything all alone.

But I also know that in order to continue to grow and challenge myself, I have to believe in my ownabilities. I still face situations that I fear are beyond my capabilities. I still have days when I feel like afraud. And I still sometimes find myself spoken over and discounted while men sitting next to me arenot. But now I know how to take a deep breath and keep my hand up. I have learned to sit at the table.